Originally Answered: What is a connecting link between aves and mammals? The connecting link is amniotesamniotesThe allantois (plural allantoides or allantoises) is a hollow sac-like structure filled with clear fluid that forms part of a developing amniote's conceptus (which consists of all embryonic and extra-embryonic tissues). It helps the embryo exchange gases and handle liquid waste.
› wiki › AllantoisAllantois - Wikipedia, which split into Saurapsids (includes birds) and Synapsids (includes mammals). The first amniotes lived about 340 million years ago.